Magnetic nanoparticles (MNPs) have an increasing presence in medicine for imaging and drug delivery applications. As newer, more advanced applications evolve there is a greater need for a better understanding of their intrinsic properties as well as improved measurement technology for researchers and application developers. The measurement of MNPs in a liquid, solid or biological matrix requires an understanding of particle composition and properties. In many biological-pharmaceutical applications of MNPs, the particle suspensions are custom designed to obtain specific performance in in-vivo situations and their intended role and effectiveness in most applications depend upon the composition of the magnetic core and the chemistry of the stabilizing outer layer, as well as their size, size distribution and concentration. As a result, there is a need for accurate measurements of the magnetic properties of MNPs in both liquid media and biological matrices with respect to size, size distribution and concentration in these matrices.